In a medium sized bowl, whisk together the cocoa powder and 1 and 1/2 cups of the powdered sugar. Set aside. Bring the heavy cream to a simmer in a small pot. Remove the cream from heat and immediately pour it into the cocoa mixture. Beat the mixture using an electric mixer for 1 minute at medium speed, until the mixture becomes thick, shiny and dark. Set aside.

In a large bowl, beat together the butter and remaining 2 cups powdered sugar until creamy in texture, about 1 minute. Then begin adding the cocoa mixture to the butter mixture, 1/4 cup at a time, and mixing well between additions. Continue to do so until you have blended all of the cocoa mixture into the butter mixture.